In variable displacement hydraulic units, especially pumps of either the single flow direction type or the reversible flow type, it is desirable to have means which maintain the position of the swashplate in a zero displacement orientation when there is no control input to move the swashplate to a stroking position.
A known hydraulic unit of the variable displacement type has a rotating cylinder block with pistons axially movable therein. The displacement of the hydraulic unit is proportional to the stroke of the pistons within the cylinder block. Where the hydraulic unit is of the axial piston type, the pistons engage a tiltable swashplate to vary the stroke of the pistons. When the swashplate is perpendicular to the axis of the cylinder block, the swashplate is in a neutral or a zero displacement orientation and the hydraulic unit has no output.
Hydraulic units of the variable displacement type commonly are employed in hydrostatic transmissions used in utility vehicles, such as garden tractors and the like. When the swashplate of the hydraulic unit is in the neutral position and the hydraulic unit has no output, the vehicle power supply is disconnected from the drive train and movement of the vehicle is prohibited. Vibration and fluid affects can result in the perturbation of the swashplate from the neutral position, however, generating a fluid displacement within the hydraulic unit. The vehicle power supply and drive are thus interconnected and the vehicle begins to creep in either a forward or reverse direction. This can be particularly dangerous in situations where the vehicle has been left unattended. It therefore is an important characteristic of a hydraulic unit that the swashplate be accurately leveled when the a condition of no output from a hydrostatic transmission is desired.
Various swashplate leveling devices have had limited success in maintaining a swashplate in its zero displacement orientation in the absence of external control forces. Forster et at U.S. Pat. No. 4,142,452, dated Mar. 6, 1979 discloses a cradle type swashplate resting in a roller bearing pocket and having four prestressed springs within the pistons of a hydraulic servo unit. In order to properly position the swashplate, very accurate spring lengths or adjustment thereof are required to minimize backlash and insure that the leveling force of a given spring does not start until the swashplate is tilted toward that spring but still assures that the spring starts to act on the swashplate exactly when the swashplate is in the zero displacement position. The use of plural springs causes problems due to the inconsistent properties of the springs.
One approach to solving the problem of inconsistent spring properties has been to provide a guide means for controlling the movement of a leveling device. U.S. Pat. No. 4,584,926 to Beck et al discloses a dual-spring biased cam having a pair of axial slots which receive pins that are fixed to a side cover for guiding the movement of the cam. Although the leveling device shown by Beck has been used for years, the design is somewhat mechanically complex and expensive to manufacture.
U.S. Pat. No. 4,845,949 to Shivvers et al shows an annular neutral mechanism which is mounted on a pump drive shaft. Because the pump shaft rotates, the mechanism is free to move relative to the shaft. In order to control the motion of the annular mechanism, a bore is formed in a pump side section which frictionally engages the outside diameter of the mechanism and limits the efficiency thereof. The Shivvers construction also requires that the neutral mechanism be positioned on the side of the swashplate opposite the rotating cylinder block. The overall length of the hydraulic unit must be appropriately increased to accommodate the travel of the neutral mechanism, resulting in a design imposing restrictive packaging problems.
A need remains for a swashplate leveling device for the swashplate of a variable displacement hydraulic unit, which is simple to manufacture and provides accurate positioning of the swashplate. The present invention is directed toward satisfying this need and overcoming one or more of the problems discussed above.